Document reading or scanning machines which are designed to optically read data from a sheet of paper commonly utilize a linear light source to project a narrow band of light across the document to be read. These optical scanning or data reading machines have found wide spread use in check reading, ballot counting, dollar changing and test scoring applications. Typically, the optical reading machine utilizes an array of light sensitive detectors, such as photo-transistors, photo-diodes, charge coupled devices, etc., to measure the amount of light reflected from or transmitted through the document. The areas of the document which contain ink or carbon penciled characters or marks will absorb light and thus will appear as dark areas to the photo-detecting elements.
In order to assure the accuracy of the optical reading machine, the light source must produce an intense, uniform light distribution across the document. Further, since the light detecting elements comprising the light sensing array, are normally silicon devices which are most sensitive to light with a wavelength centered about 1,000 nanometers, it is particularly desirable for the wavelength of the light source to be in the near infrared region.
While the conventional incandescent lamps exhibit the proper wavelength, the filament of a cylindrical elongated incandescent lamp is neither straight nor concentric and exhibits filament sag and other manufacturing alignment limitations. The utilization of such a lamp will result in the projection of the irregular filament image onto the document thus severely reducing the resolution and accuracy of the optical reading machine.